1. Field of the Invention
The present invention relates to a nitride semiconductor (for example, InXAlYGa1xe2x88x92Xxe2x88x92YN, 0xe2x89xa6X, 0xe2x89xa6Y, X+Yxe2x89xa61) device for use in a light-emitting or light-receiving device such as a light-emitting diode (LED), a laser diode (LD), a solar cell, an optical sensor, or an electronic device such as a transistor or a power device.
2. Description of the Prior Art
Nitride semiconductors are put into practical use as a material for a highly bright blue LED or a purely green LED in various light sources such as a full-color LED display, a traffic signal lamp, or an image scanner light source. Basically, these LED devices have a structure in which a buffer layer made of GaN, an n-side contact layer made of Si-doped GaN, an active layer of a single quantum well (SQW) structure made of InGaN or a multi-quantum well (MQW) structure having InGaN, a p-side cladding layer made of Mg-doped AlGaN, and a p-side contact layer made of Mg-doped GaN are successively laminated on a sapphire substrate, and show extremely excellent characteristics, namely, 5 mW with an external quantum efficiency of 9.1% in the case of a blue LED having a light-emission wavelength of 450 nm at 20 mA, and 3 mW with an external quantum efficiency of 6.3% at 20 mA in the case of a green LED having a light-emission wavelength of 520 nm.
However, although the aforesaid LED devices disclosed by the applicant of the present invention have a high output to be fully applicable for practical use and are applied to various products such as a signal, a LED device capable of reducing the consumed power without decrease in the light-emission output is desired in accordance with the requirement of energy saving and others in recent years. In order to reduce the consumed power of the LED device, reduction in the forward bias voltage of the LED device may be considered.
For example, Japanese Laid-open Patent Publication No. 8-97471 discloses a light-emitting device in which a p-type contact layer has a two-layer structure including, from the electrode side, a first layer doped with Mg at 1xc3x971020 to 1xc3x971021/cm3 and a second layer doped with Mg at a lower concentration than the first layer and within the range from 1xc3x971019 to 5xc3x971020/cm3. However, since the value of Vf attained by the technique of the aforesaid publication is 4V, a further reduction is desired.
An object of the present invention is to provide a nitride semiconductor device capable of reducing the value of Vf.
Namely, the object of the present invention can be achieved by the following construction (1) to (9).
(1) A nitride semiconductor device comprising;
a substrate,
an n-type nitride semiconductor layer formed on the substrate,
an active layer formed on the n-type nitride semiconductor layer and,
a p-type nitride semiconductor layer formed on the active layer,
wherein said active layer has a quantum well structure including a well layer made of a nitride semiconductor containing In and,
said p-type nitride semiconductor layer has a p-type contact layer, a p-type high concentration doped layer interposed between said active layer and said p-type contact layer and a p-type multi-film layer interposed between said active layer and said p-type high concentration doped layer,
said p-type multi-film layer formed by laminating alternately first nitride semiconductor layers containing Al and second nitride semiconductor layers having a different composition from said first nitride semiconductor layer, at least ones of said first nitride semiconductor layers and said second nitride semiconductor layers containing a p-type impurity,
said p-type contact layer having a p-type impurity concentration higher than that of said p-type multi-film layer and lower than that of said p-type high concentration doped layer.
(2) A nitride semiconductor device as set forth in (1), characterized in that the nitride semiconductor device further comprises;
a p-type low concentration doped layer interposed between said p-type multi-film layer and said p-type high concentration doped layer, said p-type low concentration doped layer having a p-type impurity concentration lower than that of said p-type multi-film layer.
(3) A nitride semiconductor device comprising;
a substrate,
an n-type nitride semiconductor layer formed on the substrate,
an active layer formed on the n-type nitride semiconductor layer and,
a p-type nitride semiconductor layer formed on the active layer,
wherein said active layer has a quantum well structure including a well layer made of a nitride semiconductor containing In and,
said p-type nitride semiconductor layer has a p-type contact layer, a p-type high concentration doped layer interposed between said active layer and said p-type contact layer and a p-type single film layer made of AlbGa1xe2x88x92bN (0xe2x89xa6bxe2x89xa61) containing a p-type impurity interposed between said active layer and said p-type high concentration doped layer,
said p-type contact layer having a p-type impurity concentration higher than that of said p-type single film layer and lower than that of the said p-type high concentration doped layer.
(4) A nitride semiconductor device as set forth in (3), characterized in that the nitride semiconductor device further comprises;
a p-type low concentration doped layer interposed between said p-type single film layer and said p-type high concentration doped layer, said p-type low concentration doped layer having a p-type impurity concentration lower than that of said p-type single film layer
(5) A nitride semiconductor device as set forth in (1) or (2), characterized in that said p-type multi-film layer has a p-type impurity concentration in a range from 5xc3x971017 to 1xc3x971021/cm3.
(6) A nitride semiconductor device as set forth in (3) or (4), characterized in that said p-type single film layer has a p-type impurity concentration in a range from 5xc3x971017 to 1xc3x971021/cm3.
(7) A nitride semiconductor device as set forth in any one of (1) to (6), characterized in that said p-type high concentration doped layer has a p-type impurity concentration in the range from 5xc3x971018 to 1xc3x971022/cm3.
(8) A nitride semiconductor device as set forth in any one of (1) to (7), characterized in that said p-type contact layer has a p-type impurity concentration in a range from 1xc3x971018 to 5xc3x971021/cm3.
(9) A nitride semiconductor device as set forth in any one of (2) and (4) to (8), characterized in that said p-type low concentration doped layer has a p-type impurity concentration less than 1xc3x971019/cm3.
Further, the present invention can make an improvement in the electrostatic breakdown voltage as well as reduction in Vf by the following construction (10) to (12), thereby advantageously increasing the reliability of the device.
(10) A nitride semiconductor device as set forth in any one of (1) to (9), characterized in that said n-type nitride semiconductor layer has an n-type first multi-film layer made by successive lamination of at least three layers including a lower layer made of an undoped nitride semiconductor, a middle layer made of a nitride semiconductor doped with an n-type impurity, and an upper layer made of an undoped nitride semiconductor.
(11) A nitride semiconductor device as set forth in any one of (1) to (10), characterized by having an undoped GaN layer and an n-type contact layer containing an n-type impurity that are successively formed on said substrate.
(12) A nitride semiconductor device as set forth in (11), characterized in that said n-type first multi-film layer is formed on said n-type contact layer, and further the combined thickness of said undoped GaN layer, said n-type contact layer, and said n-type first multi-film layer is 2 to 20 xcexcm.
In other words, according to the present invention, at least three p-type impurity containing layers having different p-type impurity concentrations, i.e. a p-type multi-film layer doped at a low concentration or a p-type single film layer doped at a low concentration, a p-type high concentration doped layer doped at a high concentration, and a p-type contact layer doped at a middle concentration, are successively formed as p-type nitride semiconductor layers formed on an active layer. By using these three layers having different concentrations in combination while adjusting the p-type impurity concentrations to be a low concentration, a high concentration, and a middle concentration, it is possible to provide a nitride semiconductor device capable of reducing the value of Vf.
In the aforesaid Japanese Laid-Open Patent Publication No. 8-97471, the p-type impurity concentrations of a plurality of nitride semiconductor layers formed on an active layer are made to become gradually higher in the direction towards an electrode.
In contrast, the present invention can produce a conspicuous effect by successively forming a p-type cladding layer doped at a low concentration, a p-type high concentration doped layer doped at a high concentration, and a p-type contact layer doped at a middle concentration, as described above.
In the present invention, the aforementioned terms xe2x80x9clow concentrationxe2x80x9d, xe2x80x9cmiddle concentrationxe2x80x9d, and xe2x80x9chigh concentrationxe2x80x9d refer to a relative relationship of the p-type impurity concentrations among the aforesaid three layers formed on the active layer.
Further, in the present invention, the p-type multi-film layer doped at a low concentration and the p-type single film layer doped at a low concentration are formed on an upper layer of the active layer and usually function as cladding layers, so that these layers will be hereafter described by assuming them to be p-type cladding layers. However, the p-type multi-film layer doped at a low concentration and the p-type single film layer doped at a low concentration recited in the claims are not limited to cladding layers alone.
Further, in the present invention, the p-type impurity concentration of the p-type multi-film layer made of a multi-layered film refers to an average concentration of the layers constituting the multi-layered film.
Furthermore, in the present invention, if a p-type low concentration doped layer containing a p-type impurity at a lower concentration than the p-type multi-film layer and the p-type single film layer is disposed between the p-type multi-film layer or single film layer (p-type cladding layer) and the p-type high concentration doped layer, it is preferable in view of reduction in Vf and improvement in the electrostatic breakdown voltage. An device made of a nitride semiconductor has, due to its structure, a possibility of being deteriorated even by a voltage of 100V which is far weaker than a static electricity generated in a human being. For example, there is a possibility of being deteriorated when the device is taken out from an antistatically treated bag or the like, or when it is incorporated in a product. By forming the p-type low concentration doped layer as described above, an device having a high electrostatic breakdown voltage with low Vf can be obtained, thereby increasing the reliability of the nitride semiconductor device.
Further, in the present invention, the concentration of the p-type impurity in the p-type multi-film layer is preferably 5xc3x971017 to 1xc3x971021/cm3 in view of improvement in light-emission output and reduction in Vf.
Further, in the present invention, the concentration of the p-type impurity in the p-type single film layer is preferably 5xc3x971017 to 1xc3x971021/cm3 in view of improvement in light-emission output and reduction in Vf.
Further in the present invention, the concentration of the p-type impurity in the p-type high concentration doped layer is 5xc3x971018 to 1xc3x971022/cm3 in view of reduction in Vf.
Further, in the present invention, the concentration of the p-type impurity in the p-type contact layer is preferably 1xc3x971018 to 5xc3x971021/cm3 in view of reduction in Vf.
Further, in the present invention, the concentration of the p-type impurity in the p-type low concentration doped layer is preferably less than 1xc3x971019/cm31, in view of improvement in electrostatic breakdown voltage and improvement in light-emission output.
The p-type impurity concentrations of the p-type cladding layer, the p-type high concentration doped layer, and the p-type contact layer described above are suitably selected and adjusted within the above-described ranges so that they may have a low concentration, a high concentration, and a middle concentration, respectively in their relation ship. Further, the p-type impurity concentration of the aforesaid p-type low concentration doped layer is adjusted to be contained within the above-described range so that the concentration may be further lower than the concentration of the p-type cladding layer doped at a low concentration.
Here, in the present invention, the terms xe2x80x9clow concentrationxe2x80x9d, xe2x80x9chigh concentrationxe2x80x9d, and xe2x80x9cmiddle concentrationxe2x80x9d refer to a relative relationship of the p-type impurity concentrations among the three layers including the p-type cladding layer, the p-type high concentration doped layer, and the p-type contact layer. Further, if the p-type low concentration doped layer is formed, the p-type low concentration doped layer has a concentration lower than the p-type cladding layer.
Still further, in the present invention, the n-type nitride semiconductor layer preferably has an n-type first multi-film layer made by successive lamination of at least three layers-including a lower layer made of an undoped nitride semiconductor, a middle layer made of a nitride semiconductor doped with an n-type impurity, and an upper layer made of an undoped nitride semiconductor, in view of improvement in electrostatic breakdown voltage as well as reduction in Vf by combination with the aforesaid layers on the p-side.
Still further, in the present invention, the nitride semiconductor device preferably has an n-type contact layer containing an n-type impurity and an undoped GaN layer that are formed on the substrate side of the n-type first multi-film layer successively towards the substrate, in view of further improvement in electrostatic breakdown voltage.
Still further, in the present invention, the combined thickness of the undoped GaN layer, the n-type contact layer, and the n-type first multi-film layer is preferably 2 to 20 xcexcm, more preferably 3 to 10 xcexcm, still more preferably 4 to 9 xcexcm, in view of further improvement in electrostatic breakdown voltage. Also, the thickness within the above-described range gives good device characteristics other than the electrostatic breakdown voltage. Further, the combined thickness of the aforesaid three layers are suitably adjusted within the preferable range of the thickness of each layer so that the combined thickness of the three layers may be within the above-described range.
As described above, the present invention can make further improvements in light-emission output and electrostatic breakdown voltage as well as good reduction in Vf, by combining the above-described specific three kinds of p-type layers further with the specific n-type layers, thereby increasing the reliability of the nitride semiconductor device and making it possible to widen the range of application to various products to which the nitride semiconductor device is applied.
Further, the term xe2x80x9cundopedxe2x80x9d recited in the explanation of a later-mentioned device structure refers to a layer formed without intentional doping with a impurity. Therefore, an xe2x80x9cundopedxe2x80x9d layer may refer to a layer having a impurity mixed therewith by diffusion of the impurity from an adjacent layer or by contamination from a source material or an apparatus, provided that the layer is not intentionally doped with the impurity. Here, in some cases, the impurity mixed by diffusion may have a gradient impurity concentration within the layer.
Further, in the present invention, difference in the composition refers to difference, for example, in the devices constituting the nitride semiconductor (for example, the kinds of devices in a two-device mixed crystal or a ternary compound crystal), a ratio of the elements, or a band gap energy. Further, if a specific layer is constructed with a multi-layered film, these values may refer to an average value of the whole layer.
A concrete example in which the aforesaid first nitride semiconductor layer and the aforesaid second nitride semiconductor layer have different compositions according to the present invention may be one in which the ratio of elements, the band gap energy, or the like is different as described above.
Further, in the present invention, the impurity concentration may be measured by various measuring methods, for example, by secondary ion mass spectrometry (SIMS).